1. Field of the Invention
The present invention relates to an improved MOS driver circuit for computer applications. More particularly the invention concerns a switched current mode driver designed for use in input-output channel selection functions associated with a 360/370 channel, capable of interconnection with multiple peripheral devices for receiving selection signals from the channel and redriving the signals to the next control unit.
2. Description of the Related Art
In the use of 360/370 channel systems where multiple peripheral devices must be interfaced with one or more CPU's it is common to use a control unit associated with each peripheral device being attached to the 360/370 channel. Such control units use a module that performs power supply, logic and switching functions with respect to the interface lines that are required to initiate and terminate operations between the 360/370 channel interface and the respective control units. In such systems it is usual to provide an integrated select-out bypass chip that performs select-out and select-in control for a single IBM system 360 and system 370 I/O interface. The control unit attached to the 360/370 channel interface uses a select-out signal to detect the start of a selection sequence from the channel. Unlike the other interface lines, the select-out signal is not only received by each control unit, but passes through logic and is redriven to the next control unit attached to the channel. The integrated select-out bypass function provides an electrical bypass for the select-out/select-in signal when the control unit is powered off. Besides the select out bypass function, the internal select-out bypass design contains the driver and receiver used when the control unit is powered up to receive and redrive the select-out signal to the next control unit.
Driver requirements for use in IBM 360/370 channels to drive the select-in and select-out lines require capability to drive 3.9 volts into a 95-ohm cable and be short circuit protected. Modern systems also require short circuit detection (RAS) and a high impedance mode for testing purposes. Bipolar driver circuits have been used in the past in typical line driver implementations. However the usual 6-volt power supply requirement for bipolar circuits is undesirable. Moreover, the large current limiting resistors associated with such circuits result in excessive power dissipation. Consequently, a need exists to utilize MOS circuitry for such line driver implementations in order to operate at the lower power supply voltages, typically 5 volts, available with more recently designed systems, and to reduce power dissipation.